The present invention relates generally to laser communication systems, and in particular to systems involving wavelength-division multiplexing.
In communication systems utilizing wavelength-division multiplexing (WDM), light of multiple wavelengths (actually narrow wavelength bands) propagates through a transmission medium, typically an optical fiber. Because the wavelengths are spaced apart spectrally and do not interfere with each other, they represent separate communication channels that can be independently modulated to carry information. To select a particular channel, its wavelength is extractedxe2x80x94i.e., demultiplexedxe2x80x94from the multiple-wavelength signal.
The combined WDM optical signals can be amplified as a group and transported over a single fiber to increase capacity. Each carried signal can be modulated at a different rate and in a different format (SONET, ATM, data, etc.)
Naturally, each transmitting laser in a WDM system must be configured to operate at the wavelength corresponding to its assigned channel. Ordinarily it is necessary to exert absolute frequency control over the laser sources, particularly in the case of semiconductor lasers, which possess a nominal operating frequency that is difficult to control precisely upon fabrication and which fluctuates with injection current, junction temperature and aging. Thus, an individual frequency-locked transmitter is typically used for each channel, and consequently, as the number of WDM channels increases, the transmitter farms needed to provide them become significantly more complex.
The present invention provides a transmitter capable of transmitting large numbers of WDM channels but requiring locking of only a single channel. Each of the channels can be individually modulated using an external modulator.
The transmitter is preferably a multichannel external cavity laser similar in certain respects to those described in U.S. Pat. Nos. 6,192,062 and 6,208,679, and U.S. Ser. No. 09/708,697 now U.S. Pat. No. 6,697,192 (filed on Nov. 8, 2000), the entire disclosures of which are hereby incorporated by reference. The ""697 application, for example, describes external-cavity laser designs that generate coaxially overlapping outputs at multiple wavelengths. An external laser resonator may be based on a bar of light-emitting semiconductor material whose outputs emerge from a linear sequence of stripes along the length of the bar. These outputs pass through an output-coupling lens and strike a dispersive element, such as a diffraction grating. Light dispersed by the dispersive element is reflected by a mirror back along the optical path, passing through the lens and returning to the semiconductor outputs, the opposite facets of which are partially reflective. The resulting feedback produces laser amplification, and light not reflected by the partial mirror represents discrete, spatially separate outputs.
Thus, the partially reflective semiconductor facets and the mirror together form an ensemble of individual, external-cavity lasers, each with its own optical path. The lens and dispersive element force the individual beams into a coaxial configuration, their paths intercepting at the dispersive element. Moreover, because the beam of each of these lasers strikes the dispersive element at a different angle, each laser has a different optical path and, therefore, resonates at a different wavelength. As a result, the gain elements are forced to produce rear-face outputs at the different resonance wavelengths.
The spatially separated outputs are combined by a similar optical arrangement including a coupling lens and a dispersive element. Once again the lens and dispersive element force the individual beams into a coaxial configuration, causing the different wavelengths to co-propagate. The overall result is a high-power, multi-wavelength beam with high brightness due to the coaxially overlapping component beams, and which may be focused onto the end face of an optical fiber for propagation therethrough.
In accordance with the present invention, the individual, spatially separated outputs are modulated to encode data prior to recombination. A representative laser transmitter includes a linear array of gain elements (e.g., diodes) each having a partial reflecting surface on its outer facet; an optical device (such as a collimating lens and/or a curved mirror); a dispersive element (such as a diffraction grating or prism); and a reflective device (such as a mirror) forming an external cavity. These external-cavity elements are shared by all of the resonators of all of the array elements. The laser resonator for each array element is defined by the optical path between the partial reflector and the mirror.
Fast modulation of each output is facilitated by a modulator array. This design renders the invention well-suited to WDM applications, in which it is ordinarily necessary to modulate each channel independently and desirable to modulate external to the resonator in order to achieve fast modulation rates. Intracavity modulation, which has been previously proposed for these types of sources, limits the modulation bandwidth to approximately the inverse of the cavity ring-down time.